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Abstract:

A method of polishing a substrate includes holding the substrate on a
polishing pad with a polishing head, wherein the polishing pad is
supported by a platen, creating relative motion between the substrate and
the polishing pad to polish a side of the substrate, generating a light
beam and directing the light beam towards the substrate to cause the
light beam to impinge on the side of the substrate being polished. Light
reflected from the substrate is at a detector to generate an interference
signal. A measure of uniformity is computed from the interference signal.

Claims:

1. A method of polishing a substrate, comprising:holding the substrate on
a polishing pad with a polishing head, wherein the polishing pad is
supported by a platen;creating relative motion between the substrate and
the polishing pad to polish a side of the substrate;generating a light
beam;directing the light beam towards the substrate to cause the light
beam to impinge on the side of the substrate being polished;receiving
light reflected from the substrate at a detector to generate an
interference signal; andcomputing a measure of uniformity from the
interference signal.

5. The method of claim 1, wherein computing a measure of uniformity
comprises:extracting feature information from the interference signal;
andcomputing the measure of uniformity from the extracted feature
information.

6. The method of claim 5, further comprising:comparing the measure of
uniformity to a reference; andgenerating an alert when the measure of
uniformity diverges from the reference by more than a predetermined
amount.

7. The method of claim 6, wherein the interference signal includes a low
frequency component and wherein extracting feature information
includes:measuring a first characteristic of the low frequency component;
andderiving the extracted information from the first characteristic.

8. The method of claim 7, further comprising filtering the interference
signal to obtain the low frequency component.

9. The method of claim 7, wherein the interference signal includes a high
frequency component and wherein extracting feature information
includes:measuring a second characteristic of the high frequency signal;
andderiving the extracted information from the first and second
characteristics.

10. The method of claim 9, wherein the first characteristic is an
amplitude of the high frequency signal and the second characteristic is
an amplitude of the low frequency signal, and wherein deriving the
extracted information includes computing a ratio of the amplitudes of the
high and low frequency signals.

11. The method of claim 9, further comprising filtering the interference
signal to obtain the high frequency component.

12. A method of polishing a substrate, comprising:holding the substrate on
a polishing pad with a polishing head, wherein the polishing pad is
supported by a platen;creating relative motion between the substrate and
the polishing pad to polish a side of the substrate;generating a light
beam;directing the light beam towards the substrate to cause the light
beam to impinge on the side of the substrate being polished;receiving
light reflected from the substrate at a detector to generate an
interference signal; anddisplaying a characterizing waveform for an
operator to see, the characterizing waveform presenting data for a period
of time that extends over multiple cycles of the interference signal.

15. The method of claim 14, wherein the interferometer is a laser
interferometer.

16. The method of claim 12, further comprising:receiving from an operator
input indicating two points on the waveform;computing a
frequency;generating filter coefficients from the frequency;using the
filter coefficients to generate one or both of a low frequency signal or
a high frequency signal from the interference signal; andcomputing a
measure of uniformity from one or both of the low frequency signal or the
high frequency signal.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a continuation and claims the benefit of
priority under 35 U.S.C. Section 120 of pending U.S. patent application
Ser. No. 11/532,498, filed Sep. 15, 2006, which is a continuation of U.S.
patent application Ser. No. 11/225,838, filed Sep. 12, 2005, now U.S.
Pat. No. 7,118,450, which is a continuation of U.S. patent application
Ser. No. 10/405,421, filed Apr. 1, 2003, now U.S. Pat. No. 7,011,565,
which is a continuation of U.S. patent application Ser. No. 09/863,118,
filed May 22, 2001, now U.S. Pat. No. 6,910,944, which is a continuation
of U.S. patent application Ser. No. 09/519,156, filed Mar. 6, 2000, now
U.S. Pat. No. 6,280,290, which is a continuation of U.S. patent
application Ser. No. 09/258,504, filed Feb. 26, 1999, now U.S. Pat. No.
6,045,439, which is a continuation of U.S. patent application Ser. No.
08/689,930, filed Aug. 16, 1996, now U.S. Pat. No. 5,893,796, which is a
continuation-in-part of U.S. patent application Ser. No. 08/605,769,
filed Feb. 22, 1996, now U.S. Pat. No. 5,964,643, which is a
continuation-in-part of U.S. patent application Ser. No. 08/413,982,
filed Mar. 28, 1995, abandoned. The disclosure of each prior application
is considered part of and is incorporated by reference in the disclosure
of this application.

BACKGROUND

[0002]This invention relates generally to semiconductor manufacture, and
more particularly to a method for forming a transparent window in a
polishing pad for use in chemical mechanical polishing (CMP).

[0003]In the process of fabricating modern semiconductor integrated
circuits (ICs), it is necessary to form various material layers and
structures over previously formed layers and structures. However, the
prior formations often leave the top surface topography of an in-process
wafer highly irregular, with bumps, areas of unequal elevation, troughs,
trenches, and/or other surface irregularities. These irregularities cause
problems when forming the next layer. For example, when printing a
photolithographic pattern having small geometries over previously formed
layers, a very shallow depth of focus is required. Accordingly, it
becomes essential to have a flat and planar surface, otherwise, some
parts of the pattern will be in focus and other parts will not. In fact,
surface variations on the order of less than 1000 Å over a
25×25 mm die would be preferable. In addition, if the
irregularities are not leveled at each major processing step, the surface
topography of the wafer can become even more irregular, causing further
problems as the layers stack up during further processing. Depending on
the die type and the size of the geometries involved, the surface
irregularities can lead to poor yield and device performance.
Consequently, it is desirable to effect some type of planarization, or
leveling, of the IC structures. In fact, most high density IC fabrication
techniques make use of some method to form a planarized wafer surface at
critical points in the manufacturing process.

[0004]One method for achieving semiconductor wafer planarization or
topography removal is the chemical mechanical polishing (CMP) process. In
general, the chemical mechanical polishing (CMP) process involves holding
and/or rotating the wafer against a rotating polishing platen under a
controlled pressure. As shown in FIG. 1, a typical CMP apparatus 10
includes a polishing head 12 for holding the semiconductor wafer 14
against the polishing platen 16. The polishing platen 16 is covered with
a pad 18. This pad 18 typically has a backing layer 20 which interfaces
with the surface of the platen and a covering layer 22 which is used in
conjunction with a chemical polishing slurry to polish the wafer 14.
However, some pads have only a covering layer and no backing layer. The
covering layer 22 is usually either an open cell foamed polyurethane
(e.g. Rodel IC1000) or a sheet of polyurethane with a grooved surface
(e.g. Rodel EX2000). The pad material is wetted with the chemical
polishing slurry containing both an abrasive and chemicals. One typical
chemical slurry includes KOH (Potassium Hydroxide) and fumed-silica
particles. The platen is usually rotated about its central axis 24. In
addition, the polishing head is usually rotated about its central axis
26, and translated across the surface of the platen 16 via a translation
arm 28. Although just one polishing head is shown in FIG. 1, CMP devices
typically have more than one of these heads spaced circumferentially
around the polishing platen.

[0005]A particular problem encountered during a CMP process is in the
determination that a part has been planarized to a desired flatness or
relative thickness. In general, there is a need to detect when the
desired surface characteristics or planar condition has been reached.
This has been accomplished in a variety of ways. Early on, it was not
possible to monitor the characteristics of the wafer during the CMP
process. Typically, the wafer was removed from the CMP apparatus and
examined elsewhere. If the wafer did not meet the desired specifications,
it had to be reloaded into the CMP apparatus and reprocessed. This was a
time consuming and labor-intensive procedure. Alternately, the
examination might have revealed that an excess amount of material had
been removed, rendering the part unusable. There was, therefore, a need
in the art for a device which could detect when the desired surface
characteristics or thickness had been achieved, in-situ, during the CMP
process.

[0006]Several devices and methods have been developed for the in-situ
detection of endpoints during the CMP process. For instance, devices and
methods that are associated with the use of ultrasonic sound waves, and
with the detection of changes in mechanical resistance, electrical
impedance, or wafer surface temperature, have been employed. These
devices and methods rely on determining the thickness of the wafer or a
layer thereof, and establishing a process endpoint, by monitoring the
change in thickness. In the case where the surface layer of the wafer is
being thinned, the change in thickness is used to determine when the
surface layer has the desired depth. And, in the case of planarizing a
patterned wafer with an irregular surface, the endpoint is determined by
monitoring the change in thickness and knowing the approximate depth of
the surface irregularities. When the change in thickness equals the depth
of the irregularities, the CMP process is terminated. Although these
devices and methods work reasonably well for the applications for which
they were intended, there is still a need for systems which provide a
more accurate determination of the endpoint.

SUMMARY OF THE INVENTION

[0007]In general, in one aspect, a method of polishing a substrate is
described. The substrate is held on a polishing pad with a polishing
head, wherein the polishing pad is supported by a platen. A relative
motion is created between the substrate and the polishing pad to polish a
side of the substrate. A light beam is generated. The light beam is
directed towards the substrate to cause the light beam to impinge on the
side of the substrate being polished. Light reflected from the substrate
is received at a detector to generate an interference signal. A measure
of uniformity is computed from the interference signal.

[0008]In general, in another aspect, a method of polishing a substrate is
described. The substrate is held on a polishing pad with a polishing
head, wherein the polishing pad is supported by a platen. Relative motion
is created between the substrate and the polishing pad to polish a side
of the substrate. A light beam is generated. The light beam is directed
towards the substrate to cause the light beam to impinge on the side of
the substrate being polished. Light reflected from the substrate is
received at a detector to generate an interference signal. A
characterizing waveform for an operator to see, the characterizing
waveform presenting data for a period of time that extends over multiple
cycles of the interference signal.

[0009]Additional objects and advantages of the invention will be set forth
in the description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized by means of the
instrumentalities and combinations particularly pointed out in the
claims.

BRIEF DESCRIPTION OF THE DRAWING

[0010]The accompanying drawings, which are incorporated and constitute a
part of the specification, schematically illustrate an embodiment of the
invention, and together with the general description given above and the
detailed description given below, serve to explain the principles of the
invention.

[0011]FIG. 1 is a side view of a chemical mechanical polishing (CMP)
apparatus typical of the prior art.

[0012]FIG. 2 is a side view of a chemical mechanical polishing apparatus
with endpoint detection constructed in accordance with the present
invention.

[0013]FIGS. 3A-D are simplified cross-sectional views of respective
embodiments of the window portion of the apparatus of FIG. 2.

[0014]FIG. 3E is a simplified top view of the transparent plug used in the
window portion of FIG. 3D.

[0015]FIG. 3F is a simplified cross-sectional view illustrating the
assembly of the window portion of FIG. 3D.

[0016]FIG. 4 is a simplified cross-sectional view of a window portion of
the apparatus of FIG. 2, showing components of a laser interferometer
generating a laser beam and detecting a reflected interference beam.

[0017]FIG. 5 is a simplified cross-sectional view of a blank oxide wafer
being processed by the apparatus of FIG. 2, schematically showing the
laser beam impinging on the wafer and reflection beams forming a
resultant interference beam.

[0018]FIG. 6 is a simplified top view of the platen of the apparatus of
FIG. 2, showing one possible relative arrangement between the window and
sensor flag, and the sensor and laser interferometer.

[0019]FIG. 7 is a top view of the platen of the apparatus of FIG. 2,
showing a relative arrangement between the window and sensor flag, and
the sensor and laser, where the window is in the shape of an arc.

[0020]FIG. 8 is a flow chart of a method of piece-wise data acquisition in
accordance with the present invention.

[0021]FIGS. 9A-B are graphs showing the cyclic variation in the data
signal from the laser interferometer over time during the thinning of a
blank oxide wafer. The graph of FIG. 9A shows the integrated values of
the data signal integrated over a desired sample time, and the graph of
FIG. 9B shows a filtered version of the integrated values.

[0022]FIG. 10A is a block diagram of a backward-looking method of
determining the endpoint of a CMP process to thin the oxide layer of a
blank oxide wafer in accordance with the present invention.

[0023]FIG. 10B is a block diagram of a forward-looking method of
determining the endpoint of a CMP process to thin the oxide layer of a
blank oxide wafer in accordance with the present invention.

[0024]FIGS. 11A-C are simplified cross-sectional views of a patterned
wafer with an irregular surface being processed by the apparatus of FIG.
2, wherein FIG. 11A shows the wafer at the beginning of the CMP process,
FIG. 11B shows the wafer about midway through the process, and FIG. 11C
shows the wafer close to the point of planarization.

[0025]FIG. 12 is a flow chart diagram of a method of determining the
endpoint of a CMP process to planarize a patterned wafer with an
irregular surface in accordance with the present invention.

[0026]FIG. 13 is a graph showing variation in the data signal from the
laser interferometer over time during the planarization of a patterned
wafer.

[0027]FIG. 14 is a block diagram of a method of determining the endpoint
of a CMP process to control the film thickness overlying a particularly
sized structure, or group of similarly sized structures, in accordance
with the present invention.

[0028]FIG. 15A is a simplified cross-sectional view of a wafer with a
surface imperfection being illuminated by a narrow-diameter laser beam.

[0029]FIG. 15B is a simplified cross-sectional view of a wafer with a
surface imperfection being illuminated by a wide-diameter laser beam.

[0030]FIG. 16 is a graph showing the cyclic variation in the data signal
from the laser interferometer over time during the thinning of a blank
oxide wafer and including the high frequency signal associated with a
nonuniform wafer surface.

[0031]FIG. 17 is a schematic representation of a CMP system including an
interferometer and a computer programmed to analyze and respond to the
output signal of interferometer waveform.

[0032]FIG. 18 is a block diagram showing the functionality that is
implemented within the computer to perform in-situ monitoring of
uniformity.

[0033]FIGS. 19(a)-(c) show examples of an interferometer signal, the
interferometer signal after it has been filtered by a low frequency
bandpass pass filter, and the interferometer signal after it has been
filtered by a high frequency bandpass pass filter, respectively.

[0034]FIG. 20(a)-(b) are flow charts showing the procedure for generating
and then using a signature of a CMP system to qualify it for production
use.

[0035]FIG. 21(a) is simplified cross-sectional view of an embodiment of
the window portion of the apparatus of FIG. 2 employing the polishing pad
as the window, and showing a reflection from the backside of the pad.

[0036]FIG. 21(b) is a graph showing the cyclical variation in the data
signal from the laser interferometer over time with a large DC component
caused by the reflection from the backside of the pad of the embodiment
of FIG. 21(a).

[0037]FIG. 21(c) is simplified cross-sectional view of an embodiment of
the window portion of the apparatus of FIG. 2 employing the polishing pad
as the window with a diffused backside surface to suppress reflections.

[0038]FIG. 21(d) is a graph showing the cyclical variation in the data
signal from the laser interferometer over time without the large DC
component caused by reflection from the backside of the pad as a result
of the diffuse backside surface of the embodiment of FIG. 21(c).

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0039]FIG. 2 depicts a portion of a CMP apparatus modified in accordance
with one embodiment of the present invention. A hole 30 is formed in the
platen 16 and the overlying platen pad 18. This hole 30 is positioned
such that it has a view of the wafer 14 held by a polishing head 12
during a portion of the platen's rotation, regardless of the
translational position of the head 12. A laser interferometer 32 is fixed
below the platen 16 in a position enabling a laser beam 34 projected by
the laser interferometer 32 to pass through the hole 30 in the platen 16
and strike the surface of the overlying wafer 14 during a time when the
hole 30 is adjacent the wafer 14.

[0040]A detailed view of the platen hole 30 and wafer 14 (at a time when
it overlies the platen hole 30) are shown in FIGS. 3A-C. As can be seen
in FIG. 3A, the platen hole 30 has a stepped diameter, thus forming a
shoulder 36. The shoulder 36 is used to contain and hold a quartz insert
38 which functions as a window for the laser beam 34. The interface
between the platen 16 and the insert 38 is sealed, so that the portion of
the chemical slurry 40 finding its way between the wafer 14 and insert 38
cannot leak through to the bottom of the platen 16. The quartz insert 38
protrudes above the top surface of the platen 16 and partially into the
platen pad 18. This protrusion of the insert 38 is intended to minimize
the gap between the top surface of the insert 38 and the surface of the
wafer 14. By minimizing this gap, the amount of slurry 40 trapped in the
gap is minimized. This is advantageous because the slurry 40 tends to
scatter light traveling through it, thus attenuating the laser beam
emitted from the laser interferometer 32. The thinner the layer of slurry
40 between the insert 38 and the wafer 14, the less the laser beam 34 and
light reflected from the wafer, is attenuated. It is believed a gap of
approximately 1 mm would result in acceptable attenuation values during
the CMP process. However, it is preferable to make this gap even smaller.
The gap should be made as small as possible while still ensuring the
insert 38 does not touch the wafer 14 at any time during the CMP process.
In a tested embodiment of the present invention, the gap between the
insert 38 and wafer 14 was set at 10 mils (250 μm) with satisfactory
results.

[0041]FIG. 3B shows an alternate embodiment of the platen 16 and pad 18.
In this embodiment, the quartz insert has been eliminated and no
through-hole exists in the pad 18. Instead, the backing layer 20 (if
present) of the pad 18 has been removed in the area overlying the hole 30
in the platen 16. This leaves only the polyurethane covering layer 22 of
the pad 18 between the wafer 14 and the bottom of the platen 16. It has
been found that the polyurethane material used in the covering layer 22
will substantially transmit the laser beam 34 from the laser
interferometer 32. Thus, the portion of the covering layer 22 which
overlies the platen hole 30 functions as a window for the laser beam 34.
This alternate arrangement has significant advantages. First, because the
pad 18 itself is used as the window, there is no appreciable gap.
Therefore, very little of the slurry 40 is present to cause the
detrimental scattering of the laser beam. Another advantage of this
alternate embodiment is that pad wear becomes irrelevant. In the
first-described embodiment of FIG. 3A, the gap between the quartz insert
38 and the wafer 14 was made as small as possible. However, as the pad 18
wears, this gap tends to become even smaller. Eventually, the wear could
become so great that the top surface of the insert 38 would touch the
wafer 14 and damage it. Since the pad 18 is used as the window in the
alternate embodiment of FIG. 3B, and is designed to be in contact with
the wafer 14, there are no detrimental effects due to the wearing of the
pad 18. It is noted that tests using both the opaque open-cell and
transparent grooved surface types of pads have shown that the laser beam
is less attenuated with the transparent grooved surface pad. Accordingly,
it is preferable that this type of pad be employed.

[0042]Although the polyurethane material used in the covering layer of the
pad is substantially transmissive to the laser beam, it does contain
certain additives, such as nylon microspheres, which inhibit its
transmissiveness. This problem is eliminated in the embodiment of the
invention depicted in FIG. 3C. In this embodiment, the typical pad
material in the region overlying the platen hole 30 has been replaced
with a solid polyurethane plug 42. This plug 42, which functions as the
window for the laser beam, is made of a polyurethane material which lacks
the nylon microspheres. Accordingly, the attenuation of the laser beam 34
through the plug 42 is minimized. The plug 42 may be integrally molded
into the pad 18.

[0043]For example, the plug may be formed by pouring liquid polyurethane
into a hole that has been cut in the polishing pad. The liquid
polyurethane is cured to form a plug which is integrally molded into the
polishing pad. Alternately, the plug 42 could be preformed as a solid
insert. This insert could be placed in the bulk molten polishing pad
material, and then the entire assembly could be cured so that the
material of the plug 42 and the material of the polishing pad 18 bond
together. When the assembly is cooled, the polyurethane plug 42 would be
integrally molded into the polishing pad. However, the material of the
polishing pad 18, and specifically the covering layer 22, is different
from the material of the polyurethane plug 42. Therefore when the
assembly is cured, the material of the plug 42 tends to contract and
buckle the window up or down. This causes either a cup which can
accumulate slurry or a bump which can damage the wafer 14.

[0044]Referring to FIG. 3D, in another embodiment, a two-level plug 600 is
positioned in the polishing pad 18 above the platen hole 30. The
two-level plug 600 is formed of a relatively transparent material which
acts as a window for the laser beam. The material of the two-level plug
600 may be a substantially pure polyurethane available from Rodel of
Newark, N.J., under the product name EX-2000. Such a material is
chemically inert vis-a-vis the polishing process, and erodes at the same
rate as the polishing pad. The two-level plug 600 includes an upper plug
portion 602 and a lower plug portion 604. The upper plug portion 602 fits
into a hole or opening 630 in the covering layer 22 and the lower plug
portion 604 fits into a hole or opening 632 in the backing layer 20. The
top surface 606 of the upper plug portion 602 is co-planar with the top
surface 23 of the polishing pad 18. There may be a gap 610 between the
lower surface 608 of the lower plug portion 604 and the top surface 17 of
the platen 16.

[0045]The application of a load from the wafer 14 on the polishing pad 18
will cause the backing layer 20 to compress. Thus, the width of the gap
610 will decrease. The gap 610 is selected to be sufficiently wide that
the lower surface 608 will not contact the upper surface 17 of the platen
16, even if the wafer 14 is positioned over the platen hole 30. The top
surface 606 contacts the wafer 14 but, due to the gap 610, does not exert
pressure on it. Therefore, the denser material of the two-level plug 600
does not create a locally increased load. Thus, the two-level plug 600
does not adversely affect the polishing of the wafer 14.

[0046]Referring to FIGS. 3E and 3F, the polishing pad 18 may be assembled
as follows. The two-level plug 600 is machined or molded from a solid
piece of polyurethane. An aperture 612 is cut into a polishing pad 18.
Alternately, the polishing pad 18 may be integrally molded with the
aperture 612. The aperture 612 includes two sections. The first section
of the aperture may be the hole 630 in covering layer 22 and the second
section of the aperture may be the hole 632 in the backing layer 20. The
aperture 612 matches the shape of two-level plug 600. The plug may be in
the form of adjacent rectangular slabs having different cross-sectional
areas. Specifically, the cross-sectional area of the lower plug portion
604 may be larger than the cross-sectional area of the upper plug portion
602. The upper plug portion 602 may have a length L1 of about 2.0
inches and a height H1 of about 0.5 inches The lower plug portion
604 may have a length L2 of about 2.2 inches and a height H2 of
about 0.7 inches. Thus, the lower plug portion 604 extends beyond the
upper plug portion 602 to form a rim 616 having a width W1 of about
0.1 inches. The plug may be oriented so that its longitudinal axis lies
along a radius of the polishing pad.

[0047]Although FIGS. 3D-F show the upper plug portion 602 as having a
smaller cross-sectional area than the lower plug portion 604, this is not
necessary. Instead, the upper plug portion 602 may be larger than the
lower plug portion 604. The upper plug portion 602 has a thickness
T1 equal to the thickness of covering layer 22, i.e., about fifty
mils. Thus, the thickness T1 is equal to the depth D1 of the
first section of the aperture. The lower plug portion 604 is thinner than
the backing layer 20 by about ten mils. The lower plug portion 604 may
have a thickness T2 of about forty mils. Thus, the thickness T2
is less than the depth D2 of the second section of the aperture.

[0048]An adhesive material 614 is placed on the rim 616 of the lower plug
portion 604. The adhesive material 614 may be an elastomeric polyurethane
available from Berman Industries of Van Nuys, Calif. under the trade name
WC-575 A/B. Other adhesive materials, such as rubber cement or an epoxy,
may also be used for the adhesive material 614.

[0049]An area 618 on the underside of covering layer 22 is cleaned by
scraping off any adhesive debris and washing the area with acetone. Then
the two-level plug 600 is inserted into the aperture 612 until the rim
616 of plug 600 contacts the area 618 of the polishing pad 18. This
contact area is placed under a load of approximately fifteen to twenty
pounds per square inch. This forces the adhesive material 614 into the
gaps between upper plug portion 602 and covering layer 22 or between
lower plug portion 604 and backing layer 20. After a few days at room
temperature, the adhesive material 614 will have cured and the plug 600
will be fixed in the aperture 612. The adhesive material 614 could be
cured more quickly by the application of heat, but an excessive
temperature may deform the backing material 20.

[0050]There may be grooves or pores 620 cut into the covering layer 22 of
the polishing pad 18 to provide for improved slurry distribution. These
grooves or pores 620, which are located above the lower plug portion 604,
are filled with a pure polyurethane material 622. In addition, the top
surface 606 of the two-level plug 600 is left ungrooved. Because there
are no grooves or depressions in the area of the two-level plug 600,
there is no accumulation of slurry which could block the laser beam 34.
During the conditioning process, in which a pad conditioner grinds away
the top surface 23 of the covering layer 22 to restore the roughness of
the polishing pad 18, the top surface 606 of two-level plug 600 will be
scratched and abraded. Because polyurethane is a diffusive material, the
abrasion of the top surface 606 will not significantly affect the
performance of the laser interferometer 32.

[0051]The window provided by the two-level plug 600 prevents the
accumulation of slurry above the platen hole 30 which could block the
laser beam 34. The plug 600 is formed of a material which is chemically
resistant to the slurry 40 and is chemically inert vis-a-vis the
polishing process. The plug erodes at the same rate as the rest of the
polishing pad 18. The plug is sealed within the aperture to prevent the
leakage of the slurry 40 into the platen hole 30, and the plug may be
depressed to prevent the wafer from experiencing a locally increased
load.

[0052]In operation, a CMP apparatus in accordance with the present
invention uses the laser beam from the laser interferometer to determine
the amount of material removed from the surface of the wafer, or to
determine when the surface has become planarized. The beginning of this
process will be explained in reference to FIG. 4. It is noted that a
laser and collimator 44, beam splitter 46, and detector 48 are depicted
as elements of the laser interferometer 32. This is done to facilitate
the aforementioned explanation of the operation of the CMP apparatus. In
addition, the embodiment of FIG. 3A employing the quartz insert 38 as a
window is shown for convenience. Of course, the depicted configuration is
just one possible arrangement, others can be employed. For instance, any
of the aforementioned window arrangements could be employed, and
alternate embodiments of the laser interferometer 32 are possible. One
alternate interferometer arrangement would use a laser to produce a beam
which is incident on the surface of the wafer at an angle. In this
embodiment, a detector would be positioned at a point where light
reflecting from the wafer would impinge upon it. No beam splitter would
be required in this alternate embodiment.

[0053]As illustrated in FIG. 4, the laser and collimator 44 generate a
collimated laser beam 34 which is incident on the lower portion of the
beam splitter 46. A portion of the beam 34 propagates through the beam
splitter 46 and the quartz insert 38. Once this portion of beam 34 leaves
the upper end of the insert 38, it propagates through the slurry 40, and
impinges on the surface of the wafer 14. The wafer 14, as shown in detail
in FIG. 5 has a substrate 50 made of silicon and an overlying oxide layer
52 (i.e. SiO2).

[0054]The portion of the beam 34 which impinges on the wafer 14 will be
partially reflected at the surface of the oxide layer 52 to form a first
reflected beam 54. However, a portion of the light will also be
transmitted through the oxide layer 52 to form a transmitted beam 56
which impinges on the underlying substrate 50. At least some of the light
from the transmitted beam 56 reaching the substrate 50 will be reflected
back through the oxide layer 52 to form a second reflected beam 58. The
first and second reflected beams 54, 58 interfere with each other
constructively or destructively depending on their phase relationship, to
form a resultant beam 60, where the phase relationship is primarily a
function of the thickness of the oxide layer 52.

[0055]Although, the above-described embodiment employs a silicon substrate
with a single oxide layer, those skilled in the art will recognize the
interference process would also occur with other substrates and other
oxide layers. The key is that the oxide layer partially reflects and
partially transmits, and the substrate at least partially reflects, the
impinging beam. In addition, the interference process may also be
applicable to wafers with multiple layers overlying the substrate. Again,
if each layer is partially reflective and partially transmissive, a
resultant interference beam will be created, although it will be a
combination of the reflected beams from all the layer and the substrate.

[0056]Referring again to FIG. 4, it can be seen the resultant beam 60
representing the combination of the first and second reflected beams 54,
58 (FIG. 5) propagates back through the slurry 40 and the insert 38, to
the upper portion of the beam splitter 46. The beam splitter 46 diverts a
portion of the resultant beam 60 towards the detector 48.

[0057]The platen 16 will typically be rotating during the CMP process.
Therefore, the platen hole 30 will only have a view of the wafer 14
during part of its rotation. Accordingly, the detection signal from the
laser interferometer 32 should only be sampled when the wafer 14 is
impinged by the laser beam 34. It is important that the detection signal
not be sampled when the laser beam 34 is partially transmitted through
the hole 30, as when a portion is blocked by the bottom of the platen 16
at the hole's edge, because this will cause considerable noise in the
signal. To prevent this from happening, a position sensor apparatus has
been incorporated. Any well known proximity sensor could be used, such as
Hall effect, eddy current, optical interrupter, or acoustic sensor,
although an optical interrupter type sensor was used in the tested
embodiments of the invention and will be shown in the figures that
follow. An apparatus accordingly to the present invention for
synchronizing the laser interferometer 32 is shown in FIG. 6, with an
optical interrupter type sensor 62 (e.g. LED/photodiode pair) mounted on
a fixed point on the chassis of the CMP device such that it has a view of
the peripheral edge of the platen 16. This type of sensor 62 is activated
when an optical beam it generates is interrupted. A position sensor flag
64 is attached to the periphery of the platen 16. The point of attachment
and length of the flag 64 is made such that it interrupts the sensor's
optical signal only when the laser beam 34 from the laser interferometer
32 is completely transmitted through the previously-described window
structure 66. For example, as shown in FIG. 6, the sensor 62 could be
mounted diametrically opposite the laser interferometer 32 in relation to
the center of the platen 16. The flag 64 would be attached to the platen
16 in a position diametrically opposite the window structure 66. The
length of the flag 64 would be approximately defined by the dotted lines
68, although, the exact length of the flag 64 would be fine tuned to
ensure the laser beam is completely unblocked by the platen 16 during the
entire time the flag 64 is sensed by the sensor 62. This fine tuning
would compensate for any position sensor noise or inaccuracy, the
responsiveness of the laser interferometer 32, etc. Once the sensor 62
has been activated, a signal is generated which is used to determine when
the detector signal from the interferometer 32 is to be sampled.

[0058]Data acquisition systems capable of using the position sensor signal
to sample the laser interferometer signal during those times when the
wafer is visible to the laser beam, are well known in the art and do not
form a novel part of the present invention. Accordingly, a detailed
description will not be given herein. However some considerations should
be taken into account in choosing an appropriate system. For example, it
is preferred that the signal from the interferometer be integrated over a
period of time. This integration improves the signal-to-noise ratio by
averaging the high frequency noise over the integration period. This
noise has various causes, such as vibration from the rotation of the
platen and wafer, and variations in the surface of the wafer due to
unequal planarization. In the apparatus described above the diameter of
the quartz window, and the speed of rotation of the platen, will
determine how long a period of time is available during any one rotation
of the platen to integrate the signal. However, under some circumstances,
this available time may not be adequate. For instance, an acceptable
signal-to-noise ratio might require a longer integration time, or the
interface circuitry employed in a chosen data acquisition system may
require a minimum integration time which exceeds that which is available
in one pass.

[0059]One solution to this problem is to extend the platen hole along the
direction of rotation of the platen. In other words, the window structure
66' (i.e. insert, pad, or plug) would take on the shape of an arc, as
shown in FIG. 7. Of course, the flag 64' is expanded to accommodate the
longer window structure 66'. Alternately, the window could remain the
same, but the laser interferometer would be mounted to the rotating
platen directly below the window. In this case, the CMP apparatus would
have to be modified to accommodate the interferometer below the platen,
and provisions would have to be made to route the detector signal from
the interferometer. However, the net result of either method would be to
lengthen the data acquisition time for each revolution of the platen.

[0060]Although lengthening the platen hole and window is advantageous, it
does somewhat reduce the surface area of the platen pad. Therefore, the
rate of planarization is decreased in the areas of the disk which overlie
the window during a portion of the platen's rotation. In addition, the
length of the platen hole and window must not extend beyond the edges of
the wafer, and the data sampling must not be done when the window is
beyond the edge of the wafer, regardless of the wafer's translational
position. Therefore, the length of the expanded platen hole and window,
or the time which the platen-mounted interferometer can be sampled, is
limited by any translational movement of the polishing head.

[0061]Accordingly, a more preferred method of obtaining adequate data
acquisition integration time is to collect the data over more than one
revolution of the platen. In reference to FIG. 8, during step 102, the
laser interferometer signal is sampled during the available data
acquisition time in each rotation of the platen. Next, in steps 104 and
106, each sampled signal is integrated over the aforementioned data
acquisition time, and the integrated values are stored. Then, in steps
108 and 110, a cumulative sample time is computed after each complete
revolution of the platen and compared to a desired minimum sample time.
Of course, this would constitute only one sample time if only one sample
has been taken. If the cumulative sample time equals or exceeds the
desired minimum sample time, then the stored integrated values are
transferred and summed, as shown in step 112. If not, the process of
sampling, integrating, storing, computing the cumulative sample time, and
comparing it to the desired minimum sample time continues. In a final
step 114, the summed integrated values created each time the stored
integrated values are transferred and summed, are output as a data
signal. The just-described data collection method can be implemented in a
number of well known ways, employing either logic circuits or software
algorithms. As these methods are well known, any detailed description
would be redundant and so has been omitted. It is noted that the method
of piece-wise data collection provides a solution to the problem of
meeting a desired minimum sample time no matter what the diameter of the
window or the speed of platen rotation. In fact, if the process is tied
to the position sensor apparatus, the platen rotation speed could be
varied and reliable data would still be obtained. Only the number of
platen revolutions required to obtain the necessary data would change.

[0062]The aforementioned first and second reflected beams which formed the
resultant beam 60, as shown in FIGS. 4 and 5, cause interference to be
seen at the detector 48. If the first and second beams are in phase with
each other, they cause a maxima on detector 48. Whereas, if the beams are
180 degrees out of phase, they cause a minima on the detector 48. Any
other phase relationship between the reflected beams will result in an
interference signal between the maxima and minima being seen by the
detector 48. The result is a signal output from the detector 48 that
cyclically varies with the thickness of the oxide layer 52, as it is
reduced during the CMP process. In fact, it has been observed that the
signal output from the detector 48 will vary in a sinusoidal-like manner,
as shown in the graphs of FIGS. 9A-B. The graph of FIG. 9A shows the
integrated amplitude of the detector signal (y-axis) over each sample
period versus time (x-axis). This data was obtained by monitoring the
laser interferometer output of the apparatus of FIG. 4, while performing
the CMP procedure on a wafer having a smooth oxide layer overlying a
silicon substrate (i.e. a blank oxide wafer). The graph of FIG. 9B
represents a filtered version of the data from the graph of FIG. 9A. This
filtered version shows the cyclical variation in the interferometer
output signal quite clearly. It should be noted that the period of the
interference signal is controlled by the rate at which material is
removed from the oxide layer during the CMP process. Thus, factors such
as the downward force placed on the wafer against the platen pad, and the
relative velocity between the platen and the wafer determine the period.
During each period of the output signal plotted in FIGS. 9A-B, a certain
thickness of the oxide layer is removed. The thickness removed is
proportional to the wavelength of the laser beam and the index of
refraction of the oxide layer. Specifically, the amount of thickness
removed per period is approximately λ/2n, where λ is the
freespace wavelength of the laser beam and n is the index of refraction
of the oxide layer. Thus, it is possible to determine how much of the
oxide layer is removed, in-situ, during the CMP process using the method
illustrated in FIG. 10A. First, in step 202, the number of cycles
exhibited by the data signal are counted. Next, in step 204, the
thickness of the material removed during one cycle of the output signal
is computed from the wavelength of the laser beam and the index of
refraction of the oxide layer of the wafer. Then, the desired thickness
of material to be removed from the oxide layer is compared to the actual
thickness removed, in step 206. The actual thickness removed equals the
product of the number of cycles exhibited by the data signal and the
thickness of material removed during one cycle. In the final step 208,
the CMP process is terminated whenever the removed thickness equals or
exceeds the desired thickness of material to be removed.

[0063]Alternately, less than an entire cycle might be used to determine
the amount of material removed. In this way any excess material removed
over the desired amount can be minimized. As shown in the bracketed
portions of the step 202 in FIG. 10A, the number of occurrences of a
prescribed portion of a cycle are counted in each iteration. For example,
each occurrence of a maxima (i.e. peak) and minima (i.e. valley), or vice
versa, would constitute the prescribed portion of the cycle. This
particular portion of the cycle is convenient as maxima and minima are
readily detectable via well know signal processing methods. Next, in step
204, after determining how much material is removed during a cycle, this
thickness is multiplied by the fraction of a cycle that the
aforementioned prescribed portion represents. For example in the case of
counting the occurrence of a maxima and minima, which represents one-half
of a cycle, the computed one-cycle thickness would be multiplied by
one-half to obtain the thickness of the oxide layer removed during the
prescribed portion of the cycle. The remaining steps in the method remain
unchanged. The net result of this alternate approach is that the CMP
process can be terminated after the occurrence of a portion of the cycle.
Accordingly, any excess material removed will, in most cases, be less
than it would have been if a full cycle where used as the basis for
determining the amount of material removed.

[0064]The just-described methods look back from the end of a cycle, or
portion thereof, to determine if the desired amount of material has been
removed. However, as inferred above, the amount of material removed might
exceed the desired amount. In some applications, this excess removal of
material might be unacceptable. In these cases, an alternate method can
be employed which looks forward and anticipates how much material will be
removed over an upcoming period of time and terminates the procedure when
the desired thickness is anticipated to have been removed. A preferred
embodiment of this alternate method is illustrated in FIG. 10B. As can be
seen, the first step 302 involves measuring the time between the first
occurrence of a maxima and minima, or vice versa, in the detector signal
(although an entire cycle or any portion thereof could have been
employed). Next, in step 304, the amount of material removed during that
portion of the cycle is determined via the previously described methods.
A removal rate is then calculated by dividing the amount of material
removed by the measured time, as shown in step 306. This constitutes the
rate at which material was removed in the preceding portion of the cycle.
In the next step 308, the thickness of the material removed as calculated
in step 304 is subtracted from the desired thickness to be removed to
determine a remaining removal thickness. Then, in step 310, this
remaining removal thickness is divided by the removal rate to determine
how much longer the CMP process is to be continued before its
termination.

[0065]It must be noted, however, that the period of the detector signal,
and so the removal rate, will typically vary as the CMP process
progresses. Therefore, the above-described method is repeated to
compensate for this. In other words, once a remaining time has been
calculated, the process is repeated for each occurrence of a maxima and
minima, or vice versa. Accordingly, the time between the next occurring
maxima and minima is measured, the thickness of material removed during
the portion of the cycle represented by this occurrence of the maxima and
minima (i.e. one-half) is divided by the measured time, and the removal
rate is calculated, just as in the first iteration of the method.
However, in the next step 308, as shown in brackets, the total amount of
material removed during all the previous iterations is determined before
being subtracted from the desired thickness. The rest of the method
remains the same in that the remaining thickness to be removed is divided
by the newly calculated removal rate to determine the remaining CMP
process time. In this way the remaining process time is recalculated
after each occurrence of the prescribed portion of a cycle of the
detector signal. This process continues until the remaining CMP process
time will expire before the next iteration can begin. At that point the
CMP process is terminated, as seen in step 312. Typically, the thickness
to be removed will not be accomplished in the first one-half cycle of the
detector signal, and any variation in the removal rate after being
calculated for the preceding one-half cycle will be small. Accordingly,
it is believed this forward-looking method will provide a very accurate
way of removing just the desired thickness from the wafer.

[0066]While the just-described monitoring procedure works well for the
smooth-surfaced blank oxide wafers being thinned, it has been found that
the procedure cannot be successfully used to planarize most patterned
wafers where the surface topography is highly irregular. The reason for
this is that a typical patterned wafer contains dies which exhibit a wide
variety of differently sized surface features. These differently sized
surface features tend to polish at different rates. For example, a
smaller surface feature located relatively far from other features tends
to be reduced faster than other larger features. FIG. 11A-C exemplify a
set of surface features 72, 74, 76 of the oxide layer 52 associated with
underlying structures 78, 80, 82, that might be found on a typical
patterned wafer 14, and the changes they undergo during the CMP process.
Feature 72 is a relatively small feature, feature 74 is a medium sized
feature, and feature 76 is a relatively large feature. FIG. 11A shows the
features 72, 74, 76 before polishing, FIG. 11B shows the features 72, 74,
76 about midway through the polishing process, and FIG. 11C shows the
features 72, 74, 76 towards the end of the polishing process. In FIG.
11A, the smaller feature 72 will be reduced at a faster rate than either
the medium or large features 74, 76. In addition, the medium feature 74
will be reduced at a faster rate than the large feature 76. The rate at
which the features 72, 74, 76 are reduced also decreases as the polishing
process progresses. For example, the smaller feature 72 will initially
have a high rate of reduction, but this rate will drop off during the
polishing process. Accordingly, FIG. 11B shows the height of the features
72, 74, 76 starting to even out, and FIG. 11C shows the height of the
features 72, 74, 76 essentially even. Since the differently sized
features are reduced at different rates and these rates are changing, the
interference signal produced from each feature will have a different
phase and frequency. Accordingly, the resultant interference signal,
which is partially made up of all the individual reflections from each of
the features 72, 74, 76, will fluctuate in a seemingly random fashion,
rather than the previously described periodic sinusoidal signal.

[0067]However, as alluded to above, the polishing rates of the features
72, 74, 76 tend to converge closer to the point of planarization.
Therefore, the difference in phase and frequency between the interference
beams produced by the features 72, 74, 76 tend to approach zero. This
results in the resultant interference signal becoming recognizable as a
periodic sinusoidal wave form. Therefore, it is possible to determine
when the surface of a patterned wafer has become planarized by detecting
when a sinusoidal interference signal begins. This method is illustrated
in FIG. 12. First, in step 402, a search is made for the aforementioned
sinusoidal variation in the interferometer signal. When the sinusoidal
variation is discovered, the CMP procedure is terminated, as shown in
step 404.

[0068]FIG. 13 is a graph plotting the amplitude of the detector signal
over time for a patterned wafer undergoing a CMP procedure. The sampled
data used to construct this graph was held at its previous integrated
value until the next value was reported, thus explaining the squared-off
peak values shown. A close inspection shows that a discernible sinusoidal
cycle begins to emerge at approximately 250 seconds. This coincides with
the point where the patterned wafer first became planarized. Of course,
in real-time monitoring of the interferometer's output signal, it would
be impossible to know exactly when the cycling begins. Rather, at least
some portion of the cycle must have occurred before it can be certain
that the cycling has begun. Preferably, no more than one cycle is allowed
to pass before the CMP procedure is terminated. A one-cycle limit is a
practical choice because it provides a high confidence that the cycling
has actually begun, rather than the signal simply representing variations
in the noise caused by the polishing of the differently sized features on
the surface of the wafer. In addition, the one-cycle limit ensures only a
small amount of material is removed from the surface of the wafer after
it becomes planarized. It has been found that the degree of planarization
is essentially the same after two cycles, as it was after one. Thus,
allowing the CMP procedure to continue would only serve to remove more
material from the surface of the wafer. Even though one cycle is
preferred in the case where the CMP process is to be terminated once the
patterned wafer becomes planarized, it is not intended that the present
invention be limited to that time frame. If the signal is particularly
strong, it might be possible to obtain the same level of confidence after
only a portion of a cycle. Alternately, if the signal is particularly
weak, it may take more than one cycle to obtain the necessary confidence.
The choice will depend on the characteristics of the system used. For
instance, the size of the gap between the quartz window and the surface
of the wafer will have an effect on signal strength, and so the decision
on how many cycles to wait before terminating the CMP process.

[0069]The actual determination as to when the output signal from the laser
interferometer is actually cycling, and so indicating that the surface of
the wafer has been planarized can be done in a variety of ways. For
example, the signal could be digitally processed and an algorithm
employed to make the determination. Such a method is disclosed in U.S.
Pat. No. 5,097,430, where the slope of the signal is used to make the
determination. In addition, various well known curve fitting algorithms
are available. These methods would essentially be used to compare the
interferometer signal to a sinusoidal curve. When a match occurs within
some predetermined tolerance, it is determined that the cycling has
begun. Some semiconductor applications require that the thickness of the
material overlying a structure formed on a die of a patterned wafer (i.e.
the film thickness) be at a certain depth, and that this film thickness
be repeatable from die to die, and from wafer to wafer. The previously
described methods for planarizing a typical patterned wafer will not
necessarily produce this desired repeatable film thickness. The purpose
of the planarization methods is to create a smooth and flat surface, not
to produce a particular film thickness. Accordingly, if it is desirable
to control the film thickness over a specific structure, or group of
similarly sized structures, an alternate method must be employed. This
alternate method is described below.

[0070]As alluded to previously, each differently sized surface feature
resulting from a layer of oxide being formed over a patterned structure
on a die tends to produce a reflected interference signal with a unique
frequency and phase. It is only close to the point of planarization that
the frequency and phase of each differently sized feature converges.
Prior to this convergence the unique frequency and phase of the
interference signals caused by the various differently sized features
combine to produce a detector signal that seems to vary randomly.
However, it is possible to process this signal to eliminate the
interference signal contributions of all the features being polished at
different rates, except a particularly sized feature, or group of
similarly sized features. Once the interference signal associated with
the particularly sized feature, or group of features, has been isolated,
the methods discussed in association with the removal of material from a
blank oxide disk are employed to remove just the amount of material
necessary to obtain the desired film thickness.

[0071]Of course, the frequency of the interference signal component caused
by the feature of interest must be determined prior to the signal
processing. It is believed this frequency can be easily determined by
performing a CMP process on a test specimen which includes dies
exclusively patterned with structures corresponding to the structure
which is to have a particular overlying film thickness. The detector
signal produced during this CMP process is analyzed via well known
methods to determine the unique frequency of the interference signal
caused by the surface features associated with the aforementioned
structures.

[0072]The specific steps necessary to perform the above-described method
of controlling the film thickness over a specific structure, or group of
similarly sized structures on a die, in situ, during the CMP processing
of a wafer, will now be described in reference to FIG. 14. In step 502,
the detector signal is filtered to pass only the component of the signal
having the predetermined frequency associated with the structure of
interest. This step is accomplished using well known band pass filtering
techniques. Next, in step 504 a measurement is made of the time between
the first occurrence of a maxima and minima, or vice versa, in the
detector signal (although an entire cycle or any portion thereof could
have been employed). The amount of material removed during that portion
of the cycle (i.e. one-half cycle) is determined in step 506 via
previously described methods. Then, a removal rate is then calculated by
dividing the amount of material removed by the measured time, as shown in
step 508. This constitutes the rate at which material was removed in the
preceding portion of the cycle. In the next step 510, the thickness of
the material removed as calculated in step 506 is subtracted from the
desired thickness to be removed (i.e. the thickness which when removed
will result in the desired film thickness overlying the structure of
interest), to determine a remaining removal thickness. Then, this
remaining removal thickness is divided by the aforementioned removal rate
to determine how much longer the CMP process is to be continued before it
termination, in step 512. Once a remaining time has been calculated, the
process is repeated for each occurrence of a maxima and minima, or vice
versa. Accordingly, the time between the next occurring maxima and minima
is measured, the thickness of material removed during the portion of the
cycle represented by this occurrence of the maxima and minima (i.e.
one-half) is divided by the measured time, and the removal rate is
calculated, just as in the first iteration of the method. However, in the
next step 510, as shown in brackets, the total amount of material removed
during all the previous iterations is determined before being subtracted
from the desired thickness. The rest of the method remains the same in
that the remaining thickness to be removed is divided by the newly
calculated removal rate to determine the remaining CMP process time. This
process is repeated until the remaining time expires before the next
iteration can begin. At that point, the CMP process is terminated, as
seen in step 514.

[0073]It is noted that although the method for controlling film thickness
described above utilizes the method for determining the CMP process
endpoint illustrated in FIG. 10B, any of the other endpoint determination
methods described herein could also be employed, if desired.

[0074]It is further noted that the beam diameter (i.e. spot) and
wavelength of the laser beam generated by the laser interferometer can be
advantageously manipulated. As shown in FIGS. 15A and 15B, a narrow beam
84, such as one focused to the smallest spot possible for the wavelength
employed, covers a smaller area of the surface of the wafer 14 than a
wider, less focused beam 86. This narrow beam 84 is more susceptible to
scattering (i.e. beam 88) due to surface irregularities 90, than the
wider beam 86, since the wider beam 86 spreads out over more of the
surface area of the wafer 14, and encompasses more of the surface
irregularities 90. Therefore, a wider beam 86 would have an integrating
effect and would be less susceptible to extreme variations in the
reflected interference signal, as it travels across the surface of the
wafer 14. Accordingly, a wider beam 86 is preferred for this reason. The
laser beam width can be widened using well known optical devices.

[0075]It must also be pointed out that the wider beam will reduce the
available data acquisition time per platen revolution since the time in
which the beam is completely contained within the boundaries of the
window is less than it would be with a narrower beam. However, with the
previously described methods of data acquisition, this should not present
a significant problem. In addition, since the wider beam also spreads the
light energy out over a larger area than a narrower beam, the intensity
of the reflections will be lessen somewhat. This drawback can be remedied
by increasing the power of the laser beam from the laser interferometer
so that the loss in intensity of the reflected beams is not a factor in
detection.

[0076]As for the wavelength of the laser beam, it is feasible to employ a
wavelength anywhere from the far infrared to ultraviolet. However, it is
preferred that a beam in the red light range be used. The reason for this
preference is two-fold. First, shorter wavelengths result in an increase
in the amount of scattering caused by the chemical slurry because this
scattering is proportional to the 4th power of the frequency of the laser
beam. Therefore, the longer the wavelength, the less the scattering.
However, longer wavelengths also result in more of the oxide layer being
removed per period of the interference signal, because the amount of
material removed per period equals approximately λ/2n. Therefore,
the shorter the wavelength, the less material removed in one period. It
is desirable to remove as little of the material as possible during each
period so that the possibility of any excess material being removed is
minimized. For example, in a system employing the previously described
method by which the number of cycles, or a portion thereof, are counted
to determine the thickness of the oxide layer removed, any excess
material removed over the desired amount would be minimized if the amount
of material removed during each cycle, or portion thereof, is as small as
possible.

[0077]It is believed these two competing factors in the choice of
wavelength are optimally balance if a red light laser beam is chosen. Red
light offers an acceptable degree of scattering while not resulting in an
unmanageable amount of material being removed per cycle.

FURTHER EMBODIMENTS

[0078]The generated interference waveform provides considerable additional
information about the polishing process. This additional information can
be used to provide an in-situ measurement of the uniformity of the
polished layer. It can also be used to detect when the CMP system is not
operating within spec (i.e., not operating as desired). Both of these
uses will now be described.

Uniformity Measurement:

[0079]The polishing and/or planarization operations which are performed on
the CMP system are generally required to produce a surface layer that is
uniform across the surface of the wafer/substrate. In other words, the
center of the wafer should polish at the same rate as the edge of the
wafer. Typically, the thickness of the polished layer must not vary by
more than about 5-10%. If that level of uniformity is not achieved, it is
likely that the wafer will not be usable since the device yields will be
unacceptably low. In practice, it is often quite difficult to achieve a
uniform polishing rate across the wafer. It typically requires optimizing
many different variables to keep it performing within the specs. The end
point detector described above provides a very useful tool for monitoring
the uniformity of the layer being polished and that monitoring can be
performed both in-situ data acquisition and processing.

[0080]We have discovered that the interference waveform that is produced
by the interferometer during polishing provides information about the
uniformity of the layer that is being polished. As noted above, the
output of the interferometer appear as a sinusoidal signal as the surface
layer (e.g. oxide layer) is being polished. The distance between the
peaks of that signal indicate how much material has been removed. On top
of that sinusoidal signal there will also be another higher frequency
sinusoidal signal. The amplitude of the higher frequency signal indicates
by how much the thickness of the polished layer varies across the surface
of the wafer.

[0081]The reason that the high frequency signal appears is as follows. As
the polishing is being performed, the interferometer typically samples
(or looks at) different locations across the surface of the wafer. This
is because during polishing, both the platen and the wafer are rotating
and in addition the wafer is also being moved axially relative to the
platen. Thus, during polishing different areas of the wafer's surface
pass over the hole in the platen through which the interferometer sees
the layer that is being polished. If the polished layer is completely
uniform, the resulting interference waveform will be unaffected by the
sampling of the different locations across the wafer's surface. That is,
it will have substantially the same amplitude. On the other hand, if the
polished layer is not uniform, the sampling of different locations
introduce, a further variation onto the sinusoidal base signal. This
further variation has a frequency that is dependent on the rotation and
sweep rates that are used and it has an amplitude that is proportional to
the degree of nonuniformity of the polished layer. An example of such a
waveform is shown in FIG. 16. In this particular example, the
nonuniformity was relatively large so as to clearly illustrate the high
frequency signal.

[0082]A measure of the uniformity is the ratio of the peak-to-peak
amplitude Ahf of the high frequency signal to the peak-to-peak
amplitude Alf of the low frequency signal. The smaller this ratio,
the more uniform the polished layer will be; and conversely, the larger
this ratio, the more nonuniform it will be.

[0083]A CMP system which produces a measure of uniformity is shown in FIG.
17. In addition to the components shown in the previously described FIG.
2, it also includes a computer 150, which is programmed to control the
operation of the interferometer and to perform the signal analysis that
is required to produce a measure of uniformity from the interference
signal, and it includes a display unit 160 through which various
information and results are displayed to an operator. Computer 150 can be
any device which is capable of performing the control and signal
processing functions including, for example, a standard PC which is
programmed appropriately and a dedicated, specially designed digital
processing unit. Display unit 160 can be a video display, a printer, or
any appropriate device or combination of devices for communicating
information to the operator of the CMP system.

[0084]To generate a uniformity measure, computer 150 is programmed to
implement and perform the signal processing and other functions shown in
FIG. 18. In that regard, computer 150 implements two programmable
bandpass filters, namely, a high frequency filter 152 and a low frequency
filter 154. High frequency filter 152 has a passband centered on the
frequency of the high frequency signal containing the uniformity
information and low frequency filter 154 has a passband centered on the
frequency of the low frequency signal containing the polishing rate
information. The width of both of these passbands is on the order of a
few milliherz in the case when the period is on the order of tens of
seconds. Indeed, the width of the passband is programmed to vary in
proportion with the center frequency, or stated differently, to vary
inversely to the period of the signal being examined. That is, if the
period of the relevant signal increases, the bandwidth of the passband
filter decreases and vice versa.

[0085]FIG. 19(a) shows an example of an interferometer signal obtain from
an actual system. Note that initially the signal indicates that the layer
is quite uniform, i.e., no discernible high frequency signal is riding on
top of the low frequency signal. After polishing has been performed for a
short period of time, a high frequency signal begins to appear,
indicating a certain level of nonuniformity. Low frequency filter 154
selects the low frequency component and filters out the other frequencies
to produce an output signal of the form shown in FIG. 19(b). Similarly,
high frequency filter 152 selects the high frequency component and
filters out the other frequencies to produce an output signal of the form
shown in FIG. 19(c).

[0086]Computer 150 implements two amplitude measurement functions 156 and
158 which measure the peak-to-peak amplitudes of the output signals of
filters 152 and 154, respectively. Once the amplitudes of the two
filtered signals has been determined, computer 150 computes a ratio of
the p-p amplitude of the high frequency signal to the p-p amplitude of
the low frequency signal (i.e., Ahf/Alf) (see functional block
162). After the ratio has been computed, computer 150 compares (see block
166) the computed ratio to a threshold or reference value 164 that was
previously stored in local memory. If the computed ratio exceeds the
stored threshold value, computer 150 alerts the operator that
nonuniformity of the polished layer exceeds an acceptable amount. In
response, the operator can adjust the process parameters to bring the
process back into spec.

[0087]Since the high frequency signal tends to appear only after some
polishing has been performed, it is useful to wait before attempting to
measure nonuniformity. Indeed, it may be desirable to automatically
compute the ratio periodically so as to monitor the uniformity of the
polished layer throughout the polishing operation. In that case, it may
also be desirable for computer 150 to output the computed ratios
throughout the process so that the operator can detect changes and/or
trends which are appearing in the polishing process. This would be
particularly useful if the in-situ monitoring was done during on actual
production wafers during polishing.

[0088]Note that the functions just described can be implemented through
software that is running on the computer or they can be implemented
through dedicated circuits built for this specific purpose.

[0089]The bandpass filters can be implemented using techniques which are
well known to persons skilled in the art. In the described embodiment,
they are FIR (finite impulse response) filters which can be implemented
in either the frequency or the time domain. However, to perform the
filtering in real time as the interferometer signal becomes available,
the filtering is done in the time domain by convolving the appropriate
function with the waveform as it is being generated. The appropriate
function is, of course, simply the time domain representation of a
bandpass filter having the desired characteristics (i.e., center
frequency and bandwidth).

[0090]To specify the appropriate filter parameters it is necessary to know
the frequency of the signal that is to be selected by the filter. This
information can be obtained easily from the interferometer signal
waveform(s). For example, the center frequency for the low frequency
filter can be obtained by running a batch (e.g. 25) of wafers (e.g. blank
wafers with only an oxide coating) to obtain an accurate measure of the
polishing rate. Alternatively, the polishing rate can be determined at
the start of a polishing run by measuring the distance between peaks of
the low frequency signal. Of course, using this alternative approach
produces results that are not as accurate as averaging measurements over
a larger number of wafers. In any case, the polishing rate determines the
center frequency of the bandpass filter and by knowing the center
frequency along with the desired bandwidth of the filter one can readily
determine the precise form of the time domain filter function and/or the
coefficients of the FIR filter.

[0091]The frequency of the high frequency signal can be obtained in a
similar manner, i.e., directly from the trace that is generated by the
interferometer as the CMP system is polishing the wafer. In other words,
the operator simply measures the distance between peaks of the high
frequency signal. This process can be readily automated so that the
operator, with the aid of a pointing device (e.g. a mouse), can mark two
points on the waveform appearing on a video display and the computer can
be programmed to automatically compute the frequency and then generate
the appropriate filter coefficients. The filter coefficients and/or time
domain representation of the filter functions are then stored in local
memory for use later during the polishing runs to perform the filtering
operations.

Process Signature:

[0092]The interferometer waveform also represents a signature of (i.e., it
characterizes) the system for which it was obtained. Because of this, it
provides information which is useful for qualifying a system for
production operation. If a signature is obtained for a system that is
known to be operating as desired, that signature waveform (or features
extracted from the waveform) can be used as a reference against which
subsequently generated signatures can be compared to determine whether
the system or systems from which signatures were subsequently obtained
are performing within spec. For example, if the polishing pads are
changed or a new batch of slurry is used in the CMP system, the operator
needs to know whether that change has detrimentally affected the quality
of the polishing which the system performs. We have discovered that a
change in performance of the CMP system results in a change in the
signature. That is, certain features will appear in the waveform that
were not previously present or previously existing features will change.
By detecting those changes, it is possible to detect when a system is not
performing as desired.

[0093]In the described embodiment, the extracted features from the
interferometer waveform are the polishing rate and the measure of
uniformity. Both of these characteristics are readily obtainable from the
interferometer waveform that is generated during polishing by using the
methods previously described. A properly operating system will produce a
particular polishing rate and a particular measure of uniformity. A drift
away from these reference values provides an indication that the system
is moving away from its desired operating point and alerts the operator
to the need for corrective action so as to avoid destroying product.

[0094]A method which uses a CMP system signature is illustrated in FIG.
20a and will now be described. Initially, an interferometer waveform
(i.e., a signature) is generated for a CMP system which is known to be
operating optimally (step 250). The decision as to whether the system is
operating optimally can be determined empirically by processing a set of
test wafers and analyzing the results. When the results that are produced
are within spec, then the signature can be generated for that
configuration and set of operating conditions. Before capturing a portion
of the interferometer waveform, it is desirable to polish the wafer
between 50-100% of the way through the oxide so that the waveform is
truly a signature of the polishing set up.

[0095]After the waveform has been obtained, certain relevant features are
then extracted from the generated waveform (step 252) and stored for
later use as a reference against which to evaluate that system's
performance at some later time or times (step 254). Alternatively, the
waveform itself can be stored and used as the reference. In the described
embodiment, the extracted features are the polishing rate and the measure
of uniformity, both of which can be determined from the waveform as
described above.

[0096]Referring to FIG. 20b, at some later time the stored signature (or
extracted features) can be used to qualify that system or another system
for production use. To qualify a system for production, a new signature
is obtained for that system (step 258) and the relevant features are
extracted from that new signature (step 260). The extracted features are
then compared to the stored reference set of features (step 264). If the
operating point, as characterized by the set of extracted features, falls
within a predetermined region around the reference point, as defined by
the stored reference set of features, then it is concluded that the
system is operating properly and that it can be brought online for
processing product wafers (step 266). If this process is automated, the
computer may at this point alert the operator that the process is within
spec. On the other hand, if the operating point falls outside of the
predetermined region, that is an indication that the system is not
operating within spec and the operator is alerted to this problem so that
corrective action can be taken (step 268). The corrective action might
involve adjusting some process parameter appropriately to bring the
process within spec. For example, if the polishing rate is excessive or
if oxide nonuniformity is larger than permitted, then the operator may
recognize that it is appropriate to try a new batch of slurry, or to
adjust the pressure on the pad, or to even replace the pad. The
particular course of corrective action that is chosen will of course
depend upon the details of how the system has departed from its desired
operating point, the configuration and operating parameters of the
particular system, and what the operator's experience has taught him.

[0097]To provide further useful information to the operator, the computer
also optionally outputs through its display device(s) information about
the extracted features (step 262). The displayed information may be
presented as the extracted features, the waveform, how close the various
extracted features are to the different features of the stored reference
set, or in whatever manner proves to be most useful for the operator.

[0098]Of course, the above-described in-situ, real time monitoring
procedure can be used periodically while processing production wafers or
whenever some process parameter is changed in the CMP system (e.g. a new
polishing pad is used, pad pressure is adjusted, or a new batch of slurry
is used) and it becomes necessary to know that the CMP process is still
within spec. In addition, it can be used on blank wafers, instead of
actual product, to qualify the CMP system prior to using it on actual
product.

[0099]Though we have described a straight forward and simple approach to
extracting information from the signature waveform, i.e., by using the
polishing rate and the measure of uniformity, the signature or
interferometer waveform can be analyzed by using more sophisticated
techniques (e.g. pattern or feature recognition or other image analysis
algorithms, or neural networks, just to name a few alternatives). The
information which various extracted features convey regarding the
operation of the system can be determined through experience and the ones
which convey the information that is perceived to be of most importance
to the operator can be used.

[0100]Also, it should be noted that simply displaying the interferometer
waveform (i.e., the process signature) to the operator can be yield
valuable feedback on how well the system is behaving. Typically, the
human eye is extremely sensitive in detecting even subtle changes in an
image from what one expects to see. Thus, after gaining some experience,
the operator will often be able to detect changes and imminent problems
in the overall CMP system performance simply by looking at the waveform.
Thus, in the described embodiment, the computer also displays the
signature waveform to the operator during processing so that the operator
can also use it to monitor equipment performance.

[0101]Using techniques known to persons skilled in the art, one can
readily develop software algorithms which automatically recognize or
detect the changes for which the operator is looking and which tip off
the operator to certain problems.

A Modification for Obtaining Improved Performance

[0102]Another embodiment involves a modification to the window in the pad
between the interferometer and the wafer. Although the pad will transmit
a substantial portion of the interferometer laser beam, it has been found
that there is also a significant reflective component from the bottom
surface of the pad. This situation is illustrated in FIG. 21(a) where
part of the laser beam 34 emanating from the laser interferometer 32 is
transmitted through the pad 22 to form a transmitted beam 702, and part
of the laser beam 34 is reflected from the backside surface 704 of the
pad 22 to form a reflected beam 706. The reflected beam 706 creates a
considerable direct current (DC) shift in the data signal. FIG. 21(b)
illustrates this shift (although exaggerated for purposes of clarity). In
this example, the DC shift resulting from the reflected laser light adds
about 8.0 volts to the overall signal. The DC shift creates problems in
analyzing the useful portion of the data signal. For example, if the data
analysis equipment operates in a range of 0-10 volts, amplification of
the DC shifted signal to enhance the portion of interest is all but
impossible without reducing or eliminating the DC component of the
signal. If the DC component is not eliminated, the equipment would be
saturated by the amplified signal. Reducing or eliminating the DC
component electronically requires added signal processing electronics and
may result in a degradation of the useful portion of the signal. Even if
the DC shift is not as large as described here, some signal processing
will still likely be required to eliminate it. Accordingly, a
non-electronic method of reducing or eliminating this unwanted DC
component is desirable.

[0103]It has been found that by creating a diffuse surface 704 on the
backside of the pad 22 in the area constituting the window, as depicted
in FIG. 21(c), the reflected light from that surface is attenuated. Thus,
the unwanted DC component of the data signal is reduced. The diffuse
surface 704 in effect scatters the non-transmitted laser light 708 rather
than reflecting most of it back towards the interferometer 32. The
reflected signal from the wafer must also pass through the diffuse
surface 704 and in doing so some of it will also be scattered. However,
it has been found that this does not seriously degrade the performance of
the interferometer.

[0104]FIG. 21(d) illustrates the data signal obtained when the diffuse
surface 704 is employed. As can be seen, with the elimination of the DC
component, the signal can be readily amplified and processed without the
need to electronically eliminate any DC portion.

[0105]How the diffuse surface is produced is not of central importance. It
can be produced by sanding the back surface of the polishing pad in the
vicinity of the window or by applying a material coating which is diffuse
(e.g. Scotch tape), or in any other way that produces the desired
results.

[0106]The present invention has been described in terms of a preferred
embodiment. The invention, however, is not limited to the embodiment
depicted and described. Rather, the scope of the invention is defined by
the appended claims.